The present invention relates to integrated circuit metallization structures and fabrication methods.
In modern integrated circuit fabrication, it is increasingly necessary to fill vias and contact holes which have a high xe2x80x9caspect ratioxe2x80x9d. This means a ratio of height to width which is 2:1 or more, and, as technology progresses, may be as high as 10:1 or more in future generations. Completely filling such holes with metal at an acceptably low temperature is very difficult, particularly for metals (such as aluminum and copper) which do not have a good low-temperature chemical vapor deposition (CVD) process. Moreover, even a good CVD process will not fill holes of infinitely high aspect ratios. The seamline within the cavity filled with CVD aluminum or copper will transform into a void and obstruct the current flow, resulting in a low electromigration lifetime.
Recently, contact and via filling with aluminum alloys has attracted a great deal of attention. Compared with contact/via filling with CVD tungsten, aluminum filling has the advantages of lower cost, higher yield, and potentially better electromigration resistance (since there is less flux divergence near the plug).
However, one concern with aluminum metallization is still electromigration: a pure aluminum line may gradually thin out, in service, in locations of high current density. However, the addition of copper greatly reduces this tendency. Longer electromigration (EM) lifetimes improve the product reliability. Thus, typical aluminum alloys use copper (typically one-half weight percent to one weight percent), alone or in combination with silicon (typically one-half weight percent to one weight percent), as an alloying agent. Efforts have been made to find other satisfactory aluminum alloy compositions; see e.g. Kikuta and Kikkawa, xe2x80x9cElectromigration characteristics for Alxe2x80x94Gexe2x80x94Cu,xe2x80x9d 143 J. Electrochem. Soc. 1088 (1996), which is hereby incorporated by reference.
As shown in prior art FIG. 5, a contact or via hole 502 has been etched through a dielectric layer 510 to expose an underlying layer 500, followed by the filling of the cavities 502 with a layer of aluminum or aluminum alloy 520 and the etchback (or CMP) of the aluminum layer 520 on top of the dielectric 510 to form aluminum plugs 520. As can be seen in FIG. 5, after the etchback of the aluminum layer 520, the aluminum material 520 is typically recessed 530 from the surface of the dielectric layer 510. This can undesirably result in a similar depression 550 forming in subsequently deposited metal layers 540.
Aluminum plugs may be formed by a variety of methods, including sputter-reflow, blanket CVD, selective CVD, or high pressure extrusion fill followed by an isotropic etch step or a chemical mechanical polishing (CMP) process to remove any excess aluminum. Reflow methods apply a high temperature to help newly-arrived atoms to move around on the metal surface. Extrusion cavity filling methods (like the xe2x80x9cForcefillxe2x80x9d (TM) process) apply physical pressure at high temperatures to force a soft layer of as-deposited material into the hole. The forcefill process is uniquely advantageous in filling contact or via holes with extremely high aspect ratios. Indeed, as of 1997, it appears that forcefill is the only known technique for filling holes with aspect ratios which are significantly greater than three to one.
A liner layer 505 (e.g. titanium silicide) is required for sputter-reflow, blanket CVD and high pressure extrusion fill. The liner layer 505 may also serve as a wetting layer which lowers the melting point and yield stress of the aluminum, as discussed in U.S. Provisional Patent Application Serial No. 60/037,123, filed Feb. 3, 1997, which is hereby incorporated by reference. In addition, various conductive coatings have been used on contact or via sidewalls in the prior art. For example, a barrier and adhesion layer (e.g. titanium nitride on titanium) is very commonly used. Such barrier, adhesion, and liner layers will typically be only about a few tens of nanometers thick.
In a typical CVD filling process, CVD has the disadvantage that a join 705 occurs in the middle of the cavity 720 when the cavity 720 is fully filled with CVD metal 700, which is illustrated in prior art FIG. 7A. After the metal 700 is heated, this join will become a bubble 710, as shown in prior art FIG. 7B, which increases the net series resistance of the contact or via connection. CVD aluminum processes can achieve reasonably high rates of deposition (currently up to about 200 nanometers per minute), but are typically much more expensive than sputter deposition.
In aluminum cavity-filling processes, the aluminum layer on top of the dielectric material and over the cavities is not etched back as in aluminum plug processes. As shown in prior art FIGS. 3A and 3B, the aluminum 320 is typically sputter deposited at a high temperature with a rapid deposition rate. This causes small cavities 310 to be readily bridged, with only a fairly small volume of metal 320 intruding into the cavity 310 (e.g. less than 10 percent of the volume of cavity 310), as shown in FIG. 3A. After the filling of the cavities 310 with an aluminum alloy 320 (e.g. by reflow or extrusion), a depression 330 typically forms over the cavity 310 (e.g. via, contact, or trench within a dielectric layer 300). This depression 330 is a result of mass conservation, as the aluminum alloy 320 deposited on the surface, shown in FIG. 3A, is transferred into the cavity 310, which is illustrated in FIG. 3B. The volume of the depression 330 shown in FIG. 3B typically equals the volume of the cavity 310.
A smooth surface can be achieved if the reflow or extrusion process is carried out at elevated temperature (e.g. greater than 450 degrees C.), or in an ultra-high vacuum (e.g. pressure less than 1E-8 Torr) to promote the surface diffusion of aluminum, which will smooth out the surface. However, at low temperatures (less than 450 degrees C., such as is required for use with low-k dielectrics) or in poor vacuum conditions (105 Torr or softer vacuum), the materials diffusion rate is too slow to smooth the surface, and thus a depression forms above the cavity.
These depressions undesirably are picked up as defects by defect detection tools, which increases the cycle time. A further problem with the formation of large depressions is that present lithography is unable to pattern small features above these depressions. These depressions are also undesirable for stacked via applications, because gap fill material, such as Hydrogen Silsesquioxane (HSQ) coated by spin-coating, becomes coated in the depression, making it difficult to perform subsequent via etching, since HSQ has a much slower etch rate than oxide dielectrics. Furthermore, as can be seen in prior art FIG. 4, these depressions 400 and 410 produce rough surfaces and increase the surface topography over dense cavity regions 420 and 430, because the depressions 400 and 410 above the cavities 420 and 430 overlap and compete for materials needed to fill the cavities 420 and 430, which results in incomplete filling of the cavities 420 and 430.
One conventional method of reducing the depression volume in aluminum cavity-filling processes uses a graded temperature aluminum deposition process, which is described in U.S. Pat. No. 5,108,951 to Chen et al. This process deposits a single aluminum layer, with temperature ramping, so that the aluminum is initially deposited at a low temperature, in order to reduce the likelihood of contact spiking and to begin deposition of aluminum into the cavity. Thereafter, the temperature is ramped up to a higher temperature to produce complete cavity filling and an allegedly smooth metal surface (at least for some aspect ratios). However, this process uses only a single deposition step to produce a uniform metal composition. Thus, this process does not permit the metal in the cavity or at the bottom of the cavity to be separately optimized to accommodate junction spiking considerations or increased electromigration in the cavity.
Method of Reducing the Surface Roughness
The present application discloses a method of minimizing the volume of the depressions in metal cavity filling processes. In this process, a conformal first metal layer is deposited by chemical vapor deposition, long-throw sputtering, collimated sputtering, or ionized physical vapor deposition, to partially fill the cavity. This layer is preferably deposited at a low temperature (e.g. less than 300 degrees C.) and lower deposition pressure (if deposited by sputtering). Subsequently, if high pressure extrusion/reflow is used to fill the bulk of the S cavity, a second metal layer is deposited by sputtering at temperatures greater than 350 degrees C. and at high power (e.g. greater than 10 kW) to close the mouth of cavity. The second metal layer is then preferably forced into the remaining volume of the cavity. Since part of the cavity was filled with the first metal layer before the high pressure metal extrusion/reflow, less material is required to be transported into the cavity. Therefore, a smaller depression above the cavity is produced. This method is particularly advantageous in multi-level interconnect applications involving aluminum cavity filling, but is also applicable to other metallization systems.
Advantages of the disclosed methods and structures include:
manufacturable;
existing hardware can be used for the two-step aluminum cavity filling process;
the volume of depressions is reduced, which reduces the surface topography; and
surface roughness has also been found to be reduced.